Virtual tape library

ABSTRACT

An embodiment of the invention relates to a virtual tape library apparatus providing a plurality of emulated tape storage elements. The apparatus comprises local, non-tape based storage, a store storing a data structure which associates the emulated tape storage elements with the local or a remote storage, a first interface to provide access to the local storage, in response to tape library commands identifying emulated tape storage elements associated with the local storage, a second interface to provide access to the remote storage, in response to tape library commands identifying emulated tape storage elements associated with the remote storage, and a command processor, which receives tape library commands relating to emulated storage elements, and generates respective commands for the remote storage when received library commands relate to the remote storage.

FIELD

The present invention relates to data storage, and, more particularly,to virtual or emulated tape libraries.

BACKGROUND

Media for large scale data storage can generally be classified as eithertape-based sequential access media or disk-based random access media.Storage devices which utilise tape-based media are typically used forbackup and archival storage of large datasets. Data stored on a tapemedium typically comprises backed up data files, which are structured insequential records or blocks and delimited by file mark data. Due to itssequential nature, a tape must be wound past all preceding data and filemarks in order to read from or write to any particular location or file,a process which inevitably leads to severe speed penalties compared withsystems employing disk-based random access media. Nevertheless, where aprimary requirement is for data archiving, the high capacity, longevityand relative low cost of tape media has meant that systems based ontape-based sequential access media, for example conventional tapelibraries, have remained a popular choice for many users.

A conventional tape library typically stores data on tape media, in theform of tape cartridges, and can be defined in terms of its geometry.Generally, a geometry comprises a number of addressable ‘elements’namely: storage elements (slots), for storing inactive cartridges; oneor more data transfer elements (tape drives), for reading and writingdata respectively from and to loaded cartridges; and one or more mediumtransport elements (robot pickers), for moving cartridges betweendesignated slots and drives. Some conventional tape libraries may haveadditional geometry elements such as one or more import/export elements(mailslots), for receiving new cartridges into the library or deliveringexisting cartridges out from the library.

Conventional tape libraries are typically accessed and controlled bystorage application software running on a host computer, which isattached (either directly, or indirectly via a network) to a library.The application software typically maintains a record of where in alibrary particular data has been stored, typically by reference to aparticular cartridge in a particular slot. The application software alsocontrols the library to store and retrieve data using commandsidentifying the associated geometry elements. For example, a readoperation typically involves a move medium command followed by a readcommand. The move medium command controls a picker to move a selectedcartridge (e.g. tape cartridge number 0001) stored in a particular slotto a specified drive so that particular data may be read from thecartridge and returned to the host.

Advances in hard disk storage technology have over the years reduced thedensity and cost per megabyte differential between disk media and tapemedia, thus promoting disk media as a viable alternative to tape mediain some circumstances. However, there still exist legacy data storagesoftware applications that are designed to operate withsequential-access tape devices and libraries conforming to a definedgeometry. Often, an organisation may have invested significantly inapplication software, skills and management processes for tape-basedbackup and restore activities. In some such circumstances, it has provedbeneficial to utilise disk-based random-access storage technologies instorage systems that remain compatible with legacy tape-based sequentialaccess storage applications. Storage systems of this type are known asVirtual Tape Libraries (VTL) and emulate conventional tape librariessuch that storage application software is unaware that an expectedphysical tape library has been replaced with an emulation thereof. Suchstorage systems typically replace physical tape media with one or morerandom access hard disks or disk arrays (which will hereinafter bereferred to generally as “disk storage”), translating commands based onlibrary elements into commands suitable for storing data on disk.

FIG. 1 shows an example prior art VTL in functional form. A host 101 isoperably connected to a VTL 102, which comprises an emulation module103, a file system module 104, disk storage 105 and a storage map 106,which may be stored on the disk storage 105 and loaded into volatilememory such as RAM (not shown) for quick access while the system is inoperation. Typically, the host 101 executes storage application softwareconfigured to interact with a conventional tape library of definedgeometry. When a read or write operation is initiated, the host 101sends respective tape library commands to the VTL 102 using anappropriate protocol (for example a Small Computer System Interface(SCSI) protocol), which, in turn, may be over iSCSI or Fibre Channel.The emulation module 103 receives the host commands and converts theminto commands for the file system module 104 so that the appropriatedata can be read from or written to the disk storage 105. The emulationmodule 103 converts from library commands to disk commands based on themap 106, which maps library data, received from the host 101 in terms oflibrary elements and logical locations (e.g. cartridges, records andfile marks etc.), to a data representation that can be stored on a disksubsystem (e.g. disk, volume and sector etc.).

SUMMARY

According to a first aspect, the present invention provides a virtualtape library apparatus providing a plurality of emulated tape storageelements, the apparatus comprising: local, non-tape based storage; astore storing a data structure which associates the emulated tapestorage elements with the local or remote storage; a first interface toprovide access to the local storage, in response to tape librarycommands identifying emulated tape storage elements associated with thelocal storage; a second interface to provide access to the remotestorage, in response to tape library commands identifying emulated tapestorage elements associated with the remote storage; and a commandprocessor, which receives tape library commands relating to emulatedstorage elements, and generates respective commands for the remotestorage when received library commands relate to the remote storage.

According to a second aspect, the present invention provides a virtualtape library apparatus providing a plurality of emulated tape storageelements, the apparatus comprising: local, non-tape based storage; astore storing a data structure which associates emulated tape storageelements with the local storage and indicates which, if any, of theemulated tape storage elements are readable from and/or writable to by aremote storage device; a first interface to provide access to the localstorage in response to tape library commands received from a host andidentifying emulated tape storage elements associated with the localstorage; and a second interface to provide access by a remote storagedevice to emulated tape library elements that, according to the datastructure, can be read from and/or written to by a remote storagedevice.

According to a third aspect, the present invention provides a storagesystem comprising local non-tape based storage means, means for mappinga plurality of emulated storage elements of the system to either thelocal storage means or a remote storage means, means for accessing thelocal or remote storage means according to the mapping for reading andwriting data from and to the emulated storage elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, ofwhich:

FIG. 1 is a block diagram of a prior art emulated storage device;

FIG. 2 is a block diagram of a networked computing environment whichincludes a back-up storage system;

FIG. 3 is a block diagram of two linked storage devices according to anembodiment of the present invention;

FIG. 4 is a block diagram of the functional relationship betweenaddressable emulated storage elements and local and remote emulatedstorage elements according to an embodiment of the present invention;

FIG. 5 is a flow diagram of a method of routing a request from a hostdevice;

FIG. 6 is a flow diagram of a method of requesting access to a storedcartridge on a local or remote storage device according to an embodimentof the present invention;

FIG. 7 is a block diagram showing components of a storage systemaccording to an embodiment of the invention; and

FIG. 8 is a representation of a graphical user interface for configuringan embodiment of the present invention.

DETAILED DESCRIPTION

Various embodiments of the present invention will now be described inmore detail with reference to the accompanying figures. It will beappreciated that the invention is not limited in its application, to thedetails of construction and the arrangement of components as set forthin the following description or illustrated in the drawings. It will beunderstood by a person skilled in the art that additional embodiments ofthe present invention not detailed in the description are possiblewithout departing from the scope or intention of the present invention.Accordingly, the following description should not be interpreted aslimiting in any way, and all such embodiments are intended to fallwithin scope of the present invention.

As used herein, the term “host” refers to any computer comprising atleast one processor—such as a personal computer, a workstation, amainframe or a networked client—which is capable of executing storageapplication software and communicating with other devices, such as astorage system or other computers. In addition, within this disclosure,the term “networked computer environment” includes any computingenvironment in which a plurality of hosts are mutually connected via anetwork connection using, for example, TCP/IP over Ethernet, or FibreChannel protocol layers or other suitable network technology orcombination of network technologies. Moreover, embodiments of thepresent invention may also be used in conjunction with at least onehigher level protocol layer using, for example, SCSI commands, or anyother suitable commands for communicating with a target. FIG. 2illustrates an exemplary networked computer environment comprising aplurality of hosts 201-204, a network 205 and a storage device 206. Thestorage device 206 may be connected to the network via a host 204, oralternatively (or in addition), it may be connected directly to thenetwork 205 (i.e. being network attached storage, “NAS”) via aconnection 207.

As used herein, the terms “emulated” and “virtual” are usedinterchangeably. An “emulated” device, apparatus, element, geometry orsystem denotes a device, apparatus, element, geometry or systemimplemented in software or hardware, or a combination thereof, such thatthe emulated device, apparatus, element or system behaves insubstantially the same functional manner to that of a conventional (thatis, a non-emulated) equivalent, from the point of view of a host. Forexample, whilst being implemented using different technology, a virtualtape cartridge is functionally the same as a conventional tape cartridgefrom the perspective of a connected host.

Prior art VTLs suffer from the drawback that they are often constrained,at least to some extent, by the geometry “expected” by the storageapplication software running on a host; where the expected geometry maybe referred to as a target geometry. Legacy storage application softwarerunning on one or more hosts may be tailored to specifically access alegacy tape library. Consequently, if a known VTL or a conventional tapelibrary with a different geometry is connected to the host(s), thestorage application software may require time consuming reconfiguration.For example, the “expected” number of drive and picker/robotics elementsmay be constrained, whereas the number of slot elements may not. Inaddition, configuration of a prior art VTL can be a lengthy processrequiring configuration of each element forming the emulated library,and expansion of the storage may not be possible without reconfiguringrespective host storage application software. Embodiments of the presentinvention therefore facilitate linking between a first, or localVTL—that is, the VTL that presents an emulated tape library to ahost—and a second, or remote VTL, such that remote emulated slots andmedia on the remote VTL can be accessed via the first VTL seamlessly(that is, without any indication by the first VTL of whether elementsare local or remote) by a host that is connected only to the first VTL.

To facilitate this, linking functionality is provided such that thelocal VTL may be linked to a remote VTL in order to obtain access to theadditional emulated storage elements. Linking of one or more remotestorage devices is performed within the constraints of the targetgeometry (albeit an extended target geometry) presented to the host bythe local VTL, in order to obviate reconfiguration of the host(s)'storage application software.

FIG. 3 shows the functional elements of an embodiment of the presentinvention comprising a local VTL 300 connected to a remote VTL 310. Inthis example, the VTLs are functionally the same and are connected via anetwork 350, which may be a local area network or a wide area network,although the VTLs may be connected directly to one another instead. In atypical application of the present embodiment, local and remote VTLs maybe located in different buildings, cities, countries or even continents.

A host 301 is operably connected to the local VTL 300, which comprisesan emulation module 302, a link module 304, a file system module 305,disk storage 306 and a map 307. According to the present embodiment, theemulation module 302 contains a router 303 a and a command processor 303b. The link module 304 comprises a control interface 308, and a datainterface 309 for communicating with a remote VTL 310, as will bedescribed below. The host 301 runs conventional storage applicationsoftware configured to interact with a conventional tape library ofdefined target geometry. When a read or write operation is executed, thehost 301 sends respective commands to the local VTL 300, for example,using a SCSI protocol; though, in principle, other suitable protocols,such as Fibre Channel, SATA or iSCSI, may be used. Hereafter, unlessstated otherwise, it will be assumed that the protocol referred to forcommunications between the host 301 and local VTL 300 is a standard SCSIprotocol.

In common with the local VTL 300, the remote VTL 310 comprises anemulation module 312, a link module 314, a file system module 315, diskstorage 316 and a map 317. According to the present embodiment, theemulation module 312 of the remote VTL 310 contains a router 313 a and acommand processor 313 b, and the link module 314 comprises a controlinterface 318 and a data interface 319 for communicating with the localVTL 300.

The disk storage 306 and 316 comprises one or more disk drives and/orarrays, which may comprise a plurality of commercial-off-the-shelf(COTS) disk products (and may be relatively inexpensive compared toconventional storage array products).

The host 301 communicates with the local VTL 300 using a variety ofstandard tape library commands, which may include: moving a cartridgefrom a slot to a drive (a MOVE_MEDIUM command), and reading statusinformation of the storage device (a READ_ELEMENT_STATUS command, whichreports (among other things), which elements are present at whichaddress).

With reference to the local VTL 300, the file system module 315 providesa first functional interface between the storage application softwarerunning on the host 301 and (via the emulation module 302) the diskstorage 306. The emulated library is represented in the map 307, whichis accessible to the emulation module 302. The map 307 is a datastructure that represents the emulated library in terms of one or moreaddressable elements. According to the present embodiment, the map 307contains data providing two levels of associations (or mappings) 307 aand 307 b. The map 307 provides first associations 307 a indicatingwhether emulated slots are local emulated slots, which reside on thelocal VTL 300, or remote emulated slots, which reside on the remote VTL310. Where appropriate, the mapping 307 b provides second associationsbetween data stored on local emulated cartridges of the local VTL 300and respective logical locations in the file system of the disk storage306. By ‘logical locations’ we mean, for example, disk volume, directoryname, file name etc. Optional mappings 307 c, details of which will bediscussed below, define which local emulated slots can be accessed forread and/or write access by the remote VTL 310. The file system module305 is responsible for mapping between logical file system designationsand physical locations in the disk storage 306 in a known way.

Thus, the emulation module 302 can, in response to appropriate commandsfrom the host 301, and with reference to the map 307, render or presentthe emulated library to the host as one or more addressable elements ofa conventional tape-based library.

The link module 304 provides a second functional interface between thehost 301 and (via the emulation module 302) the remote VTL 310. The linkmodule 304 provides a data interface 308, for connecting the VTL 300 tothe remote VTL 310, such that commands and data can be communicated backand forth between the local VTL 300 and the remote VTL 310. The linkmodule 304 also provides a control interface 308, which provides accessto the VTL 300 for configuration purposes, as will be described below.When linked to a remote VTL 310 via the data interface 309, commands foraccessing emulated storage elements of the remote VTL 310 arecommunicated by the link module 304 to the remote storage device usingan appropriate protocol. The access commands may be standard tapelibrary access commands or may, instead, be bespoke commands providedfor communicating between VTLs, as will be described in more detailhereafter.

The router 303 a of the emulation module 302 contains logic thatdetermines, by reference to the first associations 307 a of the map 307,whether commands received from the host 301 relate to emulated slots ofthe local VTL 300 or to emulated slots of the remote VTL 310. By virtueof the first associations 307 a and the router 303 a, the emulationmodule 302 can appropriately route commands to the local disk storage306 or to the remote VTL 310.

If no remote VTL is linked via data interface 309 to the local VTL 300,the map 307 reflects this by omitting (or in some other way renderingthe elements inaccessible) all associations with the remote VTL 310. Inthis way, the emulation module 302 presents to the host 301 only theemulated storage elements that are currently associated with the storagemedium 306 of the local VTL 300. It is possible, therefore, thatemulated storage elements that are available to the host 301 on one day(when the remote VTL 310 is attached) may not be available at anothertime (for example, if the remote VTL 310 is subsequently detached orgoes off-line).

Configuration of the VTL 300 is performed via the control interface 308.The control interface 308 typically comprises a separate physicalconnection, for example a USB connection, between the local VTL 300 anda management system 320 executing a management software application 321.In some embodiments, configuration of the local VTL 300 may be performedby a management software application running on the (or another) host301. According to the present embodiment, a user (who manages the localVTL 300) is presented by the management software application 321 with agraphical user interface (GUI) such that linking parameters may bespecified, including but not limited to specification of which remotestorage devices are linked, which media are linked, etc. Further detailsof the management software application 321 will be provided below.

In order to facilitate routing of commands from the host 301 attached tothe local VTL 300, an emulated library is represented, in terms of oneor more addressable storage elements, by a dataset forming part of thedata structure stored in the map 307 of the local VTL 300. In thisexample, the dataset defines a mapping from the addressable storageelements of the emulated tape library to respective emulated elements ofthe local VTL 300 and elements of the remote VTL 310.

FIG. 4 illustrates in graphical form an exemplary data structure showingthe addressable storage elements 401 of an emulated tape library, whichis stored in the map 307 of the local VTL 300; in particular, in themapping 307 a of map 307. In effect, the map 307 maps the emulatedelements of the emulated tape library, to the emulated elements of alocal emulated library 421 of the local VTL 300 and to the emulatedelements of a remote emulated library 441 of the remote VTL 310.

The local emulated library 421 comprises emulated elements including adrive 422, a picker 423 and six cartridge slots 424-429. The remoteemulated library 441 comprises emulated elements including two drives442 and 443, a picker 444 and four cartridge slots 445-448. In FIG. 4,it is assumed that all emulated cartridge slots of both VTLs containemulated cartridges (424-429 and 442-448), although, in alternativeembodiments, one or more slots can be empty (no cartridge containedtherein). As also shown in FIG. 4, the emulated slots of the local andremote emulated libraries each have a lock status—locked “L” or unlocked“U”—the purpose of which will be described below.

In FIG. 4, an emulated drive 402 and an emulated picker 403 map to thelocal emulated drive 422 and local emulated picker 423 respectively.Similarly, a first plurality of emulated slots 404-409 map to localemulated slots 424-429 and a second plurality of emulated slots 410 and411 map to the remote emulated slots 445 and 446 on the remote VTL 310.Thus, the emulated tape library 401 has been expanded, beyond the numberof slots available on the local VTL 300 by being linked to additionalslots from the remote emulated library of the remote VTL 310. Forsimplicity, the remote emulated slots linked to the emulated library arereported as new higher numbered slots (carrying on the from extant slotcount of the local emulated library). Thus, a host querying the geometryof the local VTL (using, for example, a READ_ELEMENT_STATUS command)would “see” a drive 402, a picker 403 and eight slots 404-411 (eachcontaining a cartridge), in accordance with the plurality of addressablestorage elements 401.

It will be appreciated that the emulated library has a geometry thatconforms to the target geometry of the local emulated library, by havingone drive 402 and one picker 423. This is despite the fact that theremote VTL 310 would normally expect to receive commands with referenceto a specified emulated slot, cartridge, drive and picker. In thepresent embodiment, the command processor 303 b of the emulator 302 inthe local VTL 300 is arranged to convert a command that is associatedwith a local emulated picker and drive, and a remote emulated slot, intoa command (intended for the remote VTL 310) referring to a remoteemulated picker, drive and slot. The command processor 303 b uses themap to re-write the commands, or generate new commands, destined for theremote VTL 310, such that the commands make sense to the remote VTL 310for its emulated geometry.

The local VTL 300 may communicate with the remote VTL 310, for data readand write operations, using standard tape library commands, whereby theremote VTL 310 can treat the local VTL as if it were a normal host. Asshown in FIG. 3, the commands are received by the remote VTL 310 via thelink module 314; however, they could equally be received via a standardhost interface if the remote VTL 310 is network attached or if the localVTL is connected to the remote VTL as a host. In any case, if the localVTL 300 communicates with the remote VTL 310 using standard tape librarycommands, the command processor 303 b only needs to translate the targetemulated element identity from that identified by the host 301 to thatrequired by the remote VTL 310. In principle, however, the two VTLscould be adapted to communicate with one another using any other kind ofappropriate protocol, and the command processor 303 b would then need tore-write library commands received from the host 301 as commandsaccording to the respective protocol. Command re-writing (ortranslation) of this kind is well known. In any case, the commandprocessor 313 b of the remote VTL 310 is arranged to receive whatevercommands are sent by the local VTL and convert them (if necessary) intocommands usable directly by the remote VTL 310 subsystems. Likewise, thecommand processor 303 b of the local VTL 300 is arranged to receiveresponses from the remote VTL 310 and convert them (if necessary) intoresponses for the host 301. Command conversion in general is well known.

One example of the mapping function described in the precedingparagraphs is illustrated in the flow diagram in FIG. 5, and isperformed by the router 303 a with reference to the map 307. By queryingthe associations contained in mappings 307 a and 307 b, the router 303 acan appropriately map commands to either the disk storage 306 of thelocal VTL 300 or to the linked remote VTL 310. According to FIG. 5, if alibrary command is received from the host 301 [step 501] and relates toreading data from an emulated cartridge in emulated slot 404, the router303 a determines from the mapping 307 a [step 502] that the emulatedslot 404 is a local emulated slot 424, which is associated with thelocal disk storage 306. The router 303 a also then determines thelogical location on the disk storage 306 of the associated data byreference to the mapping 307 b [step 503]. The emulator 302 of the localVTL 300 (the local emulator) then passes an associated disk read requestto the file system module 305 [step 504], which converts the logicallocation to a physical disk address and reads the data from the physicaladdress of the disk storage 306 [step 505]. The file system module 305returns the data to the local emulator 302 [step 506], which returns thedata to the host 301 [step 507] in a format expected by the host.Alternatively [step 502], if the library command received from the host301 relates to reading data from an emulated cartridge in emulated slot410, the router 303 a determines from the mapping 307 a that theemulated slot 410 is a remote emulated slot 445, which is associatedwith the remote VTL 310. The command processor 303 a then generates aread command for the remote VTL 310, identifying the remote emulatedslot 445 and the required data, and the emulator 302 communicates thecommand to the remote VTL 310 [step 508]. The remote VTL 310 receivesthe command and the command processor 313 b and the router 313 a of theremote VTL 310 determine the associated logical location of the data onthe remote disk storage 316, with reference to the associated mapping317 b [step 509]. The emulator 312 of the remote VTL 310 (the remoteemulator 312) then passes an associated disk read request to the filesystem module 315 [step 510], which converts the logical location to aphysical disk address and reads the data from the physical address ofthe remote disk storage 316 [step 511]. The file system module 315returns the data to the remote emulator 312 [step 512], which routes thedata back to the local emulator 302 [step 513], which in turn, by way ofthe command processor 303 b, returns the data to the host 301 [step 514]in a format expected by the host.

In the present embodiment, optional mapping 307 c of the map 307provides data defining which emulated slots of the local VTL 300 arelinkable to the remote VTL 310. This feature dictates whether linkingbetween the local VTL 300 and remote VTL 310 is uni-directional (so thatthe local VTL can link to the remote VTL but not vice versa) orbi-directional (so that each VTL can link to the other). A sub-set ofzero or more of the local emulated slots can be defined in mapping 307 cas linkable to a remote VTL 310, such that a command associated with alocal emulated slot received from the remote VTL 310 will be routed tothe appropriate emulated slot on the local VTL 300 by emulation module302 and router 303 a.

According to the present embodiment, mapping 307 c also contains datadefining a lock status (“L” designating locked and “U” designatingunlocked) of each linkable slot. In response to a command to access aparticular slot received from the host 301 or the remote VTL 310, thelock status of the associated local emulated slot is changed by theemulator 302 from unlocked to locked, during the period of the access,so that no other access request is permitted on the emulated slot. Inother words, the host is not permitted to access a local emulated slotif the remote VTL 310 is already accessing it and, likewise, the remoteVTL 310 is not permitted to access a local emulated slot if the host isalready accessing it.

As discussed, the emulated slots shown in FIG. 4 may or may not containan emulated tape cartridge at any particular time. In the embodimentdescribed, all slots contain media and the local VTL 300 linking toslots in the remote VTL 310 gains access to the emulated cartridges byselecting said linked slots. In other embodiments, a local VTL may linkto more or even all slots in a remote VTL. In either case, emulatedcartridges contained in a slot of the remote VTL, which is linked to thelocal VTL, may be included in both the emulated geometry of the localVTL and an emulated geometry of the remote device, thus being accessibleto a host or hosts connected to either of the VTLs. Alternatively,emulated cartridges contained in a slot of the remote VTL, which islinked to the local VTL, may be excluded from the emulated geometry ofthe remote VTL. It will be appreciated that, in principle, any emulatedcartridge of one VTL may be accessible to that VTL and/or to another VTLaccording to a configuration option. However, where emulated cartridgescan be accessed by more than one VTL, basic rules need to be applied towhich VTL takes precedence during a read or write operation. A simplerule is ‘first-come-first served’, whereby the first host or VTL toaccess the emulated slot obtains control until the read or writeoperation is completed and the respective emulated cartridge has beenreturned to its emulated slot.

For example, in the present embodiment, when the local VTL 300 requestsaccess to an emulated cartridge in a linked emulated slot (for exampleusing a MOVE_MEDIUM command to move the emulated cartridge to anemulated drive) of the remote VTL 310, the respective emulated slot islocked such that it can only be accessed by the local host 301 and notby any other host. Alternatively, if the required emulated slot isalready locked when a command is received, then it will not be availablefor access by the requesting host. In general, according to the presentembodiment, once locked, any emulated slot will not appear as availableto other hosts (for example in response to a READ_ELEMENT_STATUScommand), and the respective cartridge may be reported as being absentfrom the slot, for example due to its being loaded in an emulated drive.

A method of routing an access request from a local host 301 inaccordance with an embodiment of the present invention is illustrated inthe flow diagram in FIG. 6. According to FIG. 6, the local VTL 300receives a MOVE_MEDIUM request from the local host 301 [step 601]. Thelocal VTL 300 determines [step 602] if the slot associated with therequested cartridge is a local emulated slot (i.e. stored on the localdisk storage 306) or a remote emulated slot (i.e. stored on the diskstorage 316 of the remote VTL 310), by reference to the map 307. If theemulated slot is a local emulated slot, the local VTL 300 determines thelock status of the slot [step 603]. For example, emulated slot 427 isshown as locked “L” and emulated slot 428 is shown as unlocked “U”. Ifthe required slot is locked [step 604], the local VTL 300 notifies thelocal host 301 that the requested cartridge and associated slot arecurrently unavailable [step 605]. If the slot is not locked, the localVTL 300 locks the requested slot [step 606], to prevent access by otherhosts, by updating the respective lock status to “L”. Next [step 607],the local VTL 300 notifies the host 301 that the cartridge has beenmoved as requested, and is ready for a next operation by the host 301.

Alternatively, if the emulated slot is determined [step 602] to be aremote emulated slot in the remote VTL 310, the local VTL 300 sends aMOVE_MEDIUM request to the remote VTL [step 608]. The remote VTL 310receives the request [step 609] and, in response, determines if theidentified emulated slot is locked [step 610]. If the emulated slot islocked, then the remote VTL 310 reports back to the local VTL [step611], which in turn notifies the local host 301 that the requestedcartridge and associated slot are currently unavailable [step 605]. Ifthe emulated slot is not locked then the remote VTL 310 locks theemulated slot, by updating the respective lock status to “L” [step 612].Next, the remote VTL 310 notifies the local VTL 300 [step 613], and thelocal VTL 300 notifies the local host 301 [step 607], that the emulatedcartridge has been moved as requested, and is ready for a next operationby the host 301. The emulated slot, whether it is local or remote,remains locked until the local host 301 has finished carrying out theintended read or write operation and has subsequently returned theemulated cartridge to its emulated slot, at which point the lock statusis changed to unlocked and the cartridge and associated slot becomeaccessible by other hosts. In effect, according to the presentembodiment, a READ_ELEMENT_STATUS command or a MOVE_MEDIUM command canonly be successful if the target emulated cartridge is resident in itsemulated slot.

According to the present embodiment, the VTLs represented functionallyin FIG. 3 comprise the main hardware components represented in the blockdiagram in FIG. 7. Referring to FIG. 7, a VTL 700 according to thepresent embodiment includes a storage controller 704 that is connectedvia an appropriate interface 704 a to disk storage 705. The storagecontroller 704 enacts all reading and writing operations. The storagecontroller 704 is a dedicated processor, such as an ASIC, which isprogrammed to handle high speed read and write operations according tothe requirements of the VTL 700. The storage controller 704 is alsoconnected to a central processor unit (CPU) 703 and to main memory 706(such as RAM, EEPROM, Flash memory etc.). The central processor 703 is ageneral purpose programmable processor, such as an Intel Pentium™processor, which controls the overall operation of the VTL 700 and allof its components according to a control program, which is stored in andloaded from the disk storage 705 (or, alternatively, but not limited toflash memory or from elsewhere) into main memory 706 when the system isinitiated (booted). The control program code may be written in any of aplurality of suitable programming languages including Java, C++, C#, orcombinations thereof. A person skilled in the art will realise that thepresent invention is not limited to a particular programming language.Apart from storing control program instructions, the main memory 706stores the map 307 and mappings 307 a-307 c, comprising all datarepresenting the emulated library and emulated target geometry,including the associations with the local and remote emulated storageelements and the logical disk locations. The VTL 700 also includes ahost interface 702, a linked storage interface 707 and a managementinterface 708. The host interface 702 is arranged to interact with ahost 701 (directly or via a network) according to an appropriateprotocol: commands sent by a host 701 to the host interface 702 arepassed to the central processor 703, which then controls the storagecontroller 704 to enact respective read or write operations, the datafor which passes directly through the storage controller 704 and not viathe central processor 703. The linked storage interface 707 providesconnectivity (either directly or via a network) with one or more otherVTLs or other kinds of remote storage equipment. The managementinterface 708 provides connectivity (directly or via a network) with amanagement system 320 running management application software 321.

The functional elements of, for example, the local VTL 300 illustratedin FIG. 3 do not necessarily have a one-to-one correlation with thecomponents that are illustrated in FIG. 7. For example, the emulator 302may incorporate at least some aspects of the functionality of the hostinterface 702, the CPU 703, the storage controller 704 and the memory706; the actual logical arrangement if the functional blocks beingvariable and a matter of design choice. The router function 303 a andthe command processing function 303 b of the emulator 302 are, however,not speed-critical (compared with reading and writing data, which wouldbe speed-critical), and so may be embodied in software executed by theCPU 703 rather than in instructions executed by the ASIC. In any event,many different arrangements of components from that shown in FIG. 7 arepossible for implementing embodiments of the present invention and willbecome apparent to the skilled person. For example, the entirearchitecture may be implemented as a dedicated ASIC, obviating thecentral processor.

An example of a GUI for configuring a local VTL to link with a remoteVTL will now be described with reference to the diagram in FIG. 8. TheGUI 800 is generated by software and presented to a user of themanagement system 320, who wishes to configure the linkingcharacteristics of the local VTL. When the local and remote VTLs areconnected, the user is presented with the GUI 800 indicating 801 thatRemote Host 1 (in this example the remote VTL) has been detected and, inthis example, which elements have been detected. Using the emulatedmedia designations identified in FIG. 4, the emulated slots 802 of theremote VTL (Slot 445 to Slot 448), which can be linked to by the localVTL, are listed in the GUI. Against each emulated slot are three tickboxes, which are ‘ticked’ or ‘un-ticked’ by positioning a mouse pointerover the boxes and clicking a control button of the mouse in a standardway. The first ‘Link’ tick box 803 determines whether the respectivemedia item of the remote VTL should be linked to by the local VTL. Atick in the respective Link box 803 implies “yes”, whereas the absenceof a tick implies “no”. If the Link box 803 has no tick then thesubsequent boxes 804 and 805 are rendered inoperative (shown as beingblanked-out) and cannot be used. If a tick is present in the Link tickbox 803, then the next two tick boxes, a ‘Read’ tick box 804 and a‘Write’ tick box 805, can also be ticked if required. A tick in the Readtick box 804 dictates that the respective remote emulated cartridge canbe linked to, for reading purposes, by the local VTL. A tick in theWrite tick box 805 dictates that the respective remote emulatedcartridge can be linked to, for writing purposes, by the local VTL.Absence of a tick in a tick box precludes the indicated operation. Asshown in FIG. 8, emulated slots 445 and 446 can be both read from andwritten to by the local VTL, whereas emulated slot 447 can only be readfrom and emulated slot 448 cannot be read from or written to by thelocal VTL.

A second, optional aspect of the GUI 800, according to the presentembodiment, is the ability to dictate whether or not local emulatedcartridges (424-429) of the local VTL can be accessed by the remote VTL.This feature dictates whether linking is uni-directional (so that thelocal VTL can link to the remote VTL but not vice versa) orbi-directional (so that each VTL can link to the other). As shown, theuser has an option, in the form of a tick box 806, to select whether theremote VTL can link to the local VTL: a tick meaning “yes”. In no tickis added then, although not illustrated, the remainder of the GUI belowthe tick box 806 would remain inoperative to the user. However, asshown, the tick box 806 includes a tick and the GUI below remainsoperative. Each local emulated media has three associated tick boxes: aLink tick box 807, a Read tick box 808 and a Write tick box 809. Thesethree tick boxes have the same purpose as the tick boxes associated withthe remote emulated slots. As shown in FIG. 8, local emulated slots 424,425 and 426 of the local VTL are not accessible by the remote VTL,whereas slots 427, 428 and 429 are accessible by the remote VTL.However, these latter media are only accessible by the remote VTL forread operations, as only the Read tick boxes 808 are ticked. In otherwords, there is an asymmetric, bi-directional relationship between thelocal and remote VTLs, in which the local VTL can read from and write toat least some emulated media of the remote VTL but the remote VTL canonly read from selected emulated media of the local VTL.

The configuration specified by the GUI 800 is used by the managementsystem to configure the respective local VTL 300 by generating anextended emulated target geometry comprising addressable storageelements 401 for the local VTL 300, which is stored in the respectivemap 307. A similar GUI would be provided for each remote VTL (or otherstorage device) that is attached to (or can be addressed by) the localVTL. As such, individual storage elements of remote storage devices canbe selectively linked to the local VTL, in order to extend the emulatedtarget geometry of the emulated library. In addition, selected storageelements of the local VTL can be linked to (that is, made available foraccess by) remote storage devices.

In principle, the second, optional aspect of the GUI 800 may in fact bethe only configurable option of a VTL. For example, the remote VTL 310in FIG. 3 may be configurable according to the second aspect of the GUI800, to allow one or more of its emulated elements to be accessed byanother VTL, but may not support the first aspect of the GUI to permit ahost connected to it to access emulated elements of the primary VTL 300.In other words, in this case, the remote VTL 310 can only act as anon-extendable emulated library for hosts attached to it, whereas thelocal VTL 300 can be extended, for hosts attached to it, by gainingaccess to linked emulated storage elements of the remote VTL 310.

The above embodiments are to be understood as illustrative examplesonly. Further embodiments of the invention are envisaged. For example,the local and remote emulated libraries can be housed within the samedevice and/or the same non-tape-based storage media. It will beunderstood that the disk storage 306 and 316 may be replaced oraugmented by non-disk technologies and/or alternative non-sequentialstorage technologies (e.g. optical or solid state semiconductor basedstorage). According to the embodiments described herein, a local VTL islinked to a remote VTL. However a person skilled in the art willunderstand that the present invention can be adapted to interface withany suitable remote storage device, including (but not limited) to aconventional VTL or even a tape library. Moreover, embodiments of thepresent invention encompass arrangements with more than two storagedevices being mutually linked such that a host device connected to alocal VTL can seamlessly access storage elements stored on a pluralityof remote storage devices. In such an arrangement, the plurality ofdevices may be connected in a known way, for example via an Ethernetnetwork using an iSCSI protocol, to facilitate exchange of data betweendevices. Furthermore, according to other embodiments of the invention,storage elements on a local device can be duplicated to a remote devicewithout any necessary change in process at the host originating withoutany necessary change in process at the originating host.

Thus, according to some embodiments, it is possible to share an emulatedcartridge stored on a VTL such that it can effectively exist in one ormore remote emulated libraries, and be accessed by multiple remote hoststhat are attached to the remote emulated libraries. In alternativescenarios, it may be favoured to dedicate an emulated cartridge to useby only one host (which could be a local or a remote host), but permitmultiple hosts to access different emulated cartridges. Looking at thisin another way, a single host can be attached to a local VTL, accordingto an embodiment, and the VTL has flexibility to access emulatedcartridges located in other VTLs, for example, in remote locations,whereby the apparent expansion capability of the VTL is, withintheoretical bounds, limitless. This kind of solution presents a way todeal with short term storage requirement increases, which may ariseuntil a local VTL has been upgraded with additional physical storage. Inaddition, or alternatively, such an arrangement can provide a way tobalance storage requirements across an enterprise, which has multipleVTLs at different sites, but where each site has different and, perhaps,variable storage requirements. In this case, if the combined storagecapacity of the VTLs is sufficient for the needs of the enterprise, butsome sites consume more storage than others, it may be morecost-efficient to share remote emulated cartridges with thosestorage-hungry sites than to meet the actual capacity requirementsthereof. A significant advantage of embodiments of the invention is theability for a VTL to act as a slave device, by permitting access to itsstorage by another VTL, while, at the same time, remain active as amaster (or primary) device to hosts that interact therewith. In anyevent, advantageously, embodiments of the invention providesignificantly increased flexibility when optimising access to storagedistributed across a plurality linked VTL devices.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. A virtual tape library apparatus providing a plurality of emulatedtape storage elements, the apparatus comprising: local, non-tape basedstorage; a store storing a data structure which associates the emulatedtape storage elements with the local or remote storage; a firstinterface to provide access to the local storage, in response to tapelibrary commands identifying emulated tape storage elements associatedwith the local storage; a second interface to provide access to theremote storage, in response to tape library commands identifyingemulated tape storage elements associated with the remote storage; and acommand processor, which receives tape library commands relating toemulated storage elements, and generates respective commands for theremote storage in response to received library commands relating to theremote storage.
 2. Apparatus according to claim 1, wherein the datastructure includes associations between each emulated tape storageelement and either a local tape storage element, data associated withwhich is stored on the local storage, or a remote tape storage element,data associated with which is stored on the remote storage.
 3. Apparatusaccording to claim 2, wherein the command processor generates commandsrelating to remote storage elements when received library commandsrelate to the remote storage.
 4. Apparatus according to claim 2, whereinthe data structure includes associations between the data associatedwith local emulated tape storage elements and logical locations on thelocal storage.
 5. Apparatus according to claim 3, comprising a filesystem module for associating logical locations of the local storage tophysical locations of the local storage.
 6. Apparatus according to claim2, comprising a router to route commands associated with local tapestorage elements, via the first interface, to the local storage androute commands associated with the remote tape storage elements, via thesecond interface, to the remote storage.
 7. Apparatus according to claim1, comprising an emulator to receive a tape library command relating toan emulated tape storage element, and associate the tape storageelement, according to the data structure, with the respective local orremote storage.
 8. Apparatus according to claim 7, wherein the emulatorassociates a tape library command, identifying the location of data interms of emulated tape storage elements that are associated with thelocal storage, with the location of the data in terms of logicallocations on the local storage.
 9. Apparatus according to claim 1,wherein the local non-tape based storage comprises disk storage. 10.Apparatus according to claim 1, comprising a data structure to indicatewhich, if any, of the emulated tape storage elements that are associatedwith the local storage can be read from or written to by a remotestorage device.
 11. Apparatus according to claim 10, wherein the datastructure includes lock data, which is set to prevent access to anemulated tape storage element if that element is already being accessed.12. Apparatus according to claim 2, wherein the data structure isconfigurable to comprise more or fewer emulated tape storage elementsfor use by a host in dependence upon a number of remote emulated tapestorage elements that are made accessible to the apparatus.
 13. Avirtual tape library apparatus providing a plurality of emulated tapestorage elements, the apparatus comprising: local, non-tape basedstorage; a store storing a data structure which associates emulated tapestorage elements with the local storage and indicates which, if any, ofthe emulated tape storage elements are readable from and/or writable toby a remote storage device; a first interface to provide access to thelocal storage in response to tape library commands received from a hostand identifying emulated tape storage elements associated with the localstorage; and a second interface to provide access by a remote storagedevice to emulated tape library elements that, according to the datastructure, can be read from and/or written to by a remote storagedevice.
 14. Apparatus according to claim 13, wherein the data structureincludes lock data, which is set to prevent access to an emulated tapestorage element if that element is already being accessed.
 15. A storagesystem comprising local non-tape based storage means, means for mappinga plurality of emulated storage elements of the system to either thelocal storage means or a remote storage means, means for accessing thelocal or remote storage means according to the mapping for reading andwriting data from and to the emulated storage elements.